The analysis of extremely small samples of biological compounds requires ever increasing sensitivity of mass spectrometry. This is especially the case with high mass samples and MS/MS analysis. The greatest loss of sensitivity in FAB and LSIMS occurs during the sputtering conversion of sample molecules in the liquid matrix to sample ions in the vapor state. For example, the conversion efficiency of bovine insulin to ions is estimated to be between 1/300,000 and 1/3,000,000. To convert a greater percentage of sample molecules to vapor, ions rather, than vapor neutrals requires a greater energy deposition on the liquid matrix surface. The use of cluster ions instead- of the customary cesium or xenon atoms for the primary beam, provides a means of increasing the surface energy deposition and thereby sample ion conversion efficiency. The development of a cluster ion gun is proposed, based on a liquid metal ion source which is capable of producing clusters of tin with high intensity up to Sn7+ for LSIMS use. The gun will be capable of retrofitting to most existing mass spectrometers with an estimated improvement of one to two orders of magnitude in sensitivity over existing FAB or LSIMS guns.